introduce ProtocolSender interface

router/local_peer.go

@@ -250,7 +250,7 @@ func (peer *LocalPeer) handleConnectionEstablished(conn *LocalConnection) {
 
 func (peer *LocalPeer) handleSendProtocolMsg(m ProtocolMsg) {
 	peer.ForEachConnection(func(_ PeerName, conn Connection) {
-		conn.(*LocalConnection).SendProtocolMsg(m)
+		conn.(ProtocolSender).SendProtocolMsg(m)
 	})
 }
 

router/protocol.go

@@ -21,3 +21,7 @@ type ProtocolMsg struct {
 	tag ProtocolTag
 	msg []byte
 }
+
+type ProtocolSender interface {
+	SendProtocolMsg(m ProtocolMsg)
+}